Yes, I see , but I mean if I can limit sales of products. For example, I want to sell a single shirt, but when it is sold you can not buy again. This can be done with custom paypal buttons, but I don't know how to integrate it into the script. In this link you can limit the amount of products you have for sale, it could also do with your code? Thanks for the reply, it helps me a lot!!
https://www.paypal.com/cgi-bin/webscr?cmd=_button-designer&factory_type=buynow